Heart arrhythmia, such as ventricular fibrillation (VF), can be classified based upon the characteristics of the ECG waveform measured during arrhythmia. in general, the characteristics of the ECG waveform measured during arrhythmia change as the episode progresses. The changes in the characteristics of the ECG waveform indicate that there is likely an underlying physical change in the heart associated with the progression of the arrhythmia. Because of changes in the heart resulting from the progression of arrhythmia, there is a need for an electrotherapy method and apparatus that adjusts the electrotherapy applied to the patient based upon changes in the characteristics of arrhythmia.